摘要:
通过对晚白垩纪松辽盆地后金沟剖面不同类型烃源岩进行详细的有机地球化学分析,揭示嫩江组一段上部到二段下部的烃源岩有机地球化学特征及古环境意义。根据元素分析、Rock-Eval及生物标志化合物等参数,发现嫩江组一段上部随着埋深的减少,有机质的来源以湖泊内源和陆生高等植物混合为主逐渐向以湖泊内源占优势过渡;正构烷烃分布呈现出高碳数的奇偶优势与低碳数偶奇优势共存、高碳数的奇偶优势、低碳数的偶奇优势三种模式;δ13Corg、C/N及海相C30甾烷共同证实了海侵作用的发生,且24-正丙基-胆甾烷、24-异丙基-胆甾烷和甲藻甾烷含量分别为0.09~8 μg/g、0~2 μg/g、0~0.3 μg/g,随着埋深的减少呈现增加趋势。嫩江组二段下部有机质来源以湖泊内源占优势,且非海相藻类对长链正构烷烃有一定的贡献,有机质的保存与缺氧环境和微生物的活动均有关;24-正丙基-胆甾烷、24-异丙基-胆甾烷和甲藻甾烷的含量分别为2.8~12.5 μg/g、2.4~8.5 μg/g、0.1~0.64 μg/g,随埋深减少呈现一定波动性。因此认为嫩江组一段上部沉积环境总体表现为滨湖到浅湖相的过渡并且伴随海侵作用逐渐增强;嫩江组二段下部被证明是半深湖到深湖相的沉积类型,海相C30甾烷含量的波动变化指示了海侵的多期次。
Abstract:
Different types of source rock of Houjingou profile in Songliao Basin were analyzed in details for understanding the organic geochemical characteristics and the paleoenvironmental significance in Late Cretaceous, which were collected from the upper member 1 of Nenjiang Formation to the lower member 2 of the Nenjiang Formation. The results show that the source of organic matter in the upper member 1 of Nenjiang Formation changed upwardly from hybrids of lake inherent and terrestrial plants to lake endogenous predominance; There were three models in n-alkanes, the co-existence between odd-even preference of high carbon number and even-odd preference of low carbon number, odd-even preference of high carbon number, even-odd preference of low carbon number n-alkanes, respectively; The concentrations of 24-n-propylcholestane, 24-i-sopropylcholestane and dinoflagellate-steranes were 0.09 to 8 μg/g, 0 to 2 μg/g, 0 to 0.3 μg/g, respectively, increasing as the buried depth decrease, all of these indicated marine transgression combined with variations of δ13 Corg and C/N values. For the lower member 2 of the Nenjiang Formation, the source of organic matter showed lake inherent and the preservation of organic matter were related with anoxic bottom water and microorganism activities, but the non-marine algaes were also contributed to long-chain alkanes; The concentrations of 24-n-propyl cholestane, 24-i-sopropyl cholestane and dinoflagellate-steranes were 2.8 to 12.5 μg/g, 2.4 to 8.5 μg/g, 0.1 to 0.64 μg/g, respectively. Therefore, the depositional environment of upper member 1 of Nenjiang Formation showed shore lacustrine to transition of shallow lacustrine facies with increasing of marine transgression; The lower member 2 of Nenjiang Formation was turned out to be a semi-deep lake facies to deep lacustrine facies, the concentrations variation of marine algaes C30 sterane indicated that marine transgression marched more than once according to irregular fluctuate.
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